Heavy feedstocks, such as vacuum gas oils and residuum, contain relatively high concentrations of S-, N-, O-, and polynuclear aromatic containing compounds, as well as complex Ni- and V-containing compounds and asphaltenes. As a result, heavy oil is particularly difficult to upgrade in refinery operations. Metals contained in the oil tend to rapidly deactivate catalysts with which they come in contact during the upgrading process. In addition, sulfur and nitrogen are difficult to remove to the extent necessary for further processing of the upgraded products from heavy oil processing.
Furthermore, heavy oil components thermally crack during processing to form free radicals, which quickly combine to make sediment and coke precursors unless suppressed by active catalysis. During conventional hydroprocessing of heavy oils, high molecular weight coke precursors and contaminants that are deposited on catalysts quickly reduce catalytic activity.
One type of conventional heavy oil processing uses an ebullated bed system, in which the catalyst is maintained in a fluidized state within the reaction zone. At periodic intervals, a portion of the fluidized bed of catalyst, along with a small portion of fluidizing liquid, is removed from the system. A comparable amount of catalyst is added to the system to maintain a constant quantity of catalyst in the system at any one time.
In conventional heavy oil upgrading, e.g., using an ebullated bed system that relies solely on a conventional pelletized hydroprocessing catalyst, poorly converted or unconverted feed may precipitate as sediment or sludge. Sediment can then plug equipment leading to shorter runtime and/or operational issues, as well as poor product quality. The formation of sediment or sludge typically increases with conversion and feed difficulty. For this reason, the conversion or ability to process a flexible array of feeds is limited in these units.
Thus, there is a need for improved hydroprocessing catalyst systems that are more efficient, permit an increase in conversion and/or the use of a wider range of feedstocks in a cost-effective manner, as compared with the prior art. There is a further need for methods of introducing a co-catalyst into a hydroprocessing unit.